Species-habitat interrelations: Swainson's nest in trees along cropfields, rivers or prairies and feed in these habitats *04,09,10,14*. Savannas are important to this species in Illinois *15*. Swainson's is a rare breeding season resident *01*; the habitats discussed are those required in the nesting season.

Comments on food habits: General: Although some sources cite insects (primarily orthoptera) and cold- blooded vertebrates as important food items *04,05,08,12,18*, most recent literature suggests small mammals are most important prey *09, 10,14*.Juvenile: No comments.Adult: No comments.

Comments on environmental associations:General: Swainson's hawk lives in a variety of open lands and woodlands; seemingly available nest trees are the only limiting factor *09*.Feeding juvenile: No comments.Resting juvenile: No comments.Feeding adult: No comments.Resting adult: No comments.Breeding adult: No comments.

Reproduction: Hawks return to breeding grounds (males first *12*) in U.S. mid-Mar. To mid-Apr. *08*. Mates of same color phase are preferred *06*. Territory selection by nesting pairs begins in Apr.*07*. If male dies, female will replace mate in same breeding season *12*. Male presumably begins nest building, female may assist *12*. Perhaps 50% of nests are built new each season *10*. Nest built in trees along rivers, agricultural fields, forest edges, usually 3.5-6.5 m high. No apparent preference for species of nest tree *08,09,10*. Clutch of 2-4 eggs, ave. 2-3 *04,6,12*; both parents (mostly female) incubate *04,06,08* about 28 days *04,05*. A second clutch may be laid if first is lost *06*. Young hatch at 1-2 d. intervals *08*, in mid-June in IL. *13*. Parents are not defensive of nest until after hatching *13*; even then, Swainsons are not particularly aggressive around the nest and may even allow some other bird species to nest in lower parts of their nest *08*. Nestling period 30-40 days *04,05,07*. After fledging, young begin their hunting on the ground, presumably for insects *05*. Post-fledging period 4-4.5 wk. *12*.

Behavior: Hunts from perches (trees, telephone poles, fence posts, knolls), also by soaring. Usually pounces on prey on the ground, may also eat insects and birds in the air *04,08*. Reported nesting densities (area/pr.): 6.7 sq. km. *06* and 2.4 sq. mi. *07* In wy. Home ranges: 900 ha. in WA *14*, 670 ha. in WY *06*, 2000 ha. in ND *10*. Mean distance between ND nests 2.3 km. *10*. Some references suggest Swainson's not strongly territorial or aggressive *05,08*. Conversely, a Canadian study reports aggressive encounters between nesting Swainson's hawks and ferruginous and red- tailed hawks, Swainson's being the aggressor *09*. Great horned owls and red-tailed hawks may occupy Swainson's nests - both species nest earlier *13*. Longest migration of all N. American hawks. Migrate in large flocks, often flying in concentrations along the same narrow corridors each yr. *04*. It is thought they do not eat or drink during migration *04*. Most winter in S. America *04*; a small population winters in FL and TX *20*.

Limiting factors: Interspecific competition: great horned owls and red-tailed hawks occupy Swainson's nests and nesting habitats *13*. Swainson's reproductive success decreases with decreasing distance between their nests and those of ferruginous and red-tailed hawks *09*. Disease is not an important cause of nestling mortality *07*. About 33% of nests lost are destroyed in bad weather, 20% by predation, 3% by human factors (44% unknown) in one study *10* 1/3 of all nests may be lost *06*. Human disturbance may be an important factor reducing productivity in IL *15*.

Population parameters: In two WY studies: egg loss 15% and 2.06 Young fledged/nesting pr. *06*; 40% nestling mortality and 43% fledging success in another *07* suggests a much lower productivity. In IL, productivity is seemingly declining, and Swainson's populations are considered "sporatic and declining in numbers" *15*. Longevity in wild 13 yr. *12*.

22. U.S. DEPT. OF INTERIOR, FISH AND WILDLIFE SERVICE [USDI]. 1982. ENDAN- GERED AND THREATENED WILDLIFE AND PLANTS; REVIEW OF VERTEBRATE WILD- LIFE FOR LISTING AS ENDANGERED AND THREATENED SPECIES. FED. REG. 47(251): 58454-58460.